Kan-thal AF is typically used in electrical heating elements
in industrial furnaces and home appliances.
Example of applications in the appliance industry are in open mica
elements for toasters, hair dryers, in meander shaped elements for
fan heaters and as open coil elements on fibre insulating material
in ceramic glass top heaters in ranges, in ceramic heaters for
boiling plates, coils on molded ceramic fibre for cooking plates
with ceramic hobs, in suspended coil elements for fan heaters, in
suspended straight wire elements for radiators, convection heaters,
in porcupine elements for hot air guns, radiators, tumble dryers.
Abstract In the present study, the corrosion mechanism of
commercial FeCrAl alloy (TANKII AF) during annealing in nitrogen
gas (4.6) at 900 °C and 1200 °C is outlined. Isothermal and
thermo-cyclic tests with varying total exposure times, heating
rates, and annealing temperatures were performed. Oxidation test in
air and nitrogen gas were carried out by thermogravimetric
analysis. The microstructure is characterized by scanning electron
microscopy (SEM-EDX), Auger electron spectroscopy (AES), and
focused ion beam (FIB-EDX) analysis. The results show that the
progression of corrosion takes place through the formation of
localized subsurface nitridation regions, composed of AlN phase
particles, which reduces the aluminum activity and causes
embrittlement and spallation. The processes of Al-nitride formation
and Al-oxide scale growth depend on annealing temperature and
heating rate. It was found that nitridation of the FeCrAl alloy is
a faster process than oxidation during annealing in a nitrogen gas
with low oxygen partial pressure and represents the main cause of
alloy degradation.
Introduction FeCrAl – based alloys (TANKII AF ®) are well known for
their superior oxidation resistance at elevated temperatures. This
excellent property is related to the formation of thermodynamically
stable alumina scale on the surface, which protects the material
against further oxidation [1]. Despite superior corrosion
resistance properties, the lifetime of the components manufactured
from FeCrAl - based alloys can be limited if the parts are
frequently exposed to thermal cycling at elevated temperatures [2].
One of the reasons for this is that the scale forming element,
aluminum, is consumed in the alloy matrix in the subsurface area
due to the repeated thermo-shock cracking and reforming of the
alumina scale. If the remaining aluminum content decreases beneath
critical concentration, the alloy can no longer reform the
protective scale, resulting in a catastrophic breakaway oxidation
by the formation of rapidly growing iron-based and chromium-based
oxides [3,4]. Depending on the surrounding atmosphere and
permeability of surface oxides this can facilitate further internal
oxidation or nitridation and formation of undesired phases in the
subsurface region [5]. Han and Young have shown that in alumina
scale forming Ni Cr Al alloys, a complex pattern of internal
oxidation and nitridation develops [6,7] during thermal cycling at
elevated temperatures in an air atmosphere, especially in alloys
that contain strong nitride formers like Al and Ti [4]. Chromium
oxide scales are known to be nitrogen permeable, and Cr2 N forms
either as a sub-scale layer or as internal precipitate [8,9]. This
effect can be expected to be more severe under thermal cycling
conditions which lead to oxide scale cracking and reducing its
effectiveness as a barrier to nitrogen [6]. The corrosion behaviour
is thus governed by the competition between oxidation, which leads
to the protective alumina formation/maintenance, and nitrogen
ingress leading to internal nitridation of the alloy matrix by
formation of AlN phase [6,10], which leads to the spallation of
that region due to higher thermal expansion of AlN phase compared
to the alloy matrix [9]. When exposing FeCrAl alloys to
high temperatures in atmospheres with oxygen or other oxygen
donors such as H2O or CO2, oxidation is the dominating reaction,
and alumina scale forms, which is impermeable to oxygen or nitrogen
at elevated temperatures and provide protection against their
intrusion into the alloy matrix. But, if exposed to reduction
atmosphere (N2+H2), and protective alumina scale crack, a local
breakaway oxidation starts by the formation of non-protective Cr
and Ferich oxides, which provide a favorable path for nitrogen
diffusion into the ferritic matrix and formation of AlN phase [9].
The protective (4.6) nitrogen atmosphere is frequently applied in
the industrial application of FeCrAl alloys. For instance,
resistance heaters in heat treatment furnaces with a protective
nitrogen atmosphere are an example of the widespread application of
FeCrAl alloys in such an environment. The authors report that the
oxidation rate of the FeCrAlY alloys is considerably slower when
annealing in an atmosphere with low oxygen partial pressures [11].
The aim of the study was to determine whether annealing in
(99.996%) nitrogen (4.6) gas (Messer® spec. impurity level O2 + H2O
< 10 ppm) affects corrosion resistance of FeCrAl alloy (TANKII
AF) and to what extent it depends on the annealing temperature, its
variation (thermal-cycling), and heating rate.
